1. Field of the Invention
The invention relates to a device for determining the flow of a flowing medium according to the float principle. Here, the device has at least one floating body and at least one measuring tube with medium flowing through it. The floating body is movable in the measuring tube along an axis.
2. Description of Related Art
Measuring devices that determine the flow of a medium—particularly a fluid, such as liquids or gases—by means of the float process have, in most cases, a measuring tube that is arranged vertically—i.e., in the direction of gravity—or is alternatively arranged horizontally. The measuring tube, through which a medium to be measured flows, usually has a conical interior that widens in the flow direction, in which the movable floating body is arranged along the direction of flow.
The flowing medium—in the case of a vertically oriented measuring tube—exerts a force in the flow direction on the floating body, which is dependent on the flow resistance of the floating body. Additionally, the buoyancy of the floating body acts in the direction of flow. The weight force acts opposite the direction of flow, so that three forces act on the floating body: two in the direction of flow and one in the opposite direction. According to transient response, the floating body remains at a level in which the three forces are just balanced. Since the position is dependent on this, i.e., the position of the floating body in the measuring tube is dependent on the flow of the medium, this value allows for conclusions about the flow.
If the measuring tube is arranged horizontally, the lack of a weight force is replaced by the spring force of a spring that acts on the floating body.
The height can be determined, for example, in that the floating body is arranged in an inspection glass, which is provided with a corresponding scale. It is described in patent DE 196 39 060 C2 that the determining of position occurs via a permanent magnet connected to the floating body.
The flow of the medium is obtained from the determined height dependent on a flow coefficient α. The flow coefficient α is dependent on the geometric shapes of the measuring tube and the floating body and on the Ruppel number Ru, which incorporates medium-specific parameters such as viscosity and density (see, for example, German Utility Model DE 83 17 576 U1). For the implementation of the measuring principle, a data set with the flow coefficient α is determined depending on the height h and the Ruppel number Ru for the medium to be measured or for a plurality of possible media and stored for evaluation, thus, specifically for each measuring device type, i.e., for identical instruments.
The disadvantage is that many media have a viscosity that is dependent on temperature. For most liquids, the viscosity decreases with increasing temperature, so that less friction force can be transferred to the floating body. As a result, the floating body moves toward its rest position, which would be associated with reduced flow.
A relatively complex design of a variable-area flowmeter can be seen in German Patent Application DE 33 41 460 A1, in which the media is partly lead through a bypass that is opened and closed by a valve assembly dependent on temperature.